Many types of electrochemical storage devices containing zinc electrodes are known and have been practiced in the art. These electrochemical energy storage devices may be electrically or mechanically rechargeable (secondary batteries) or not rechargeable (primary batteries). Typical zinc electrodes used in these devices include nickel-zinc, silver-zinc, zinc-oxygen, zinc-air, zinc-mercuric-oxide, zinc-carbon and zinc-manganese-oxide.
In rechargeable batteries, the zinc electrode exhibits a relatively short charge/discharge cycle life due to the solubility of the zinc in the alkaline battery electrolyte. During battery charge/discharge cycling, zinc is dissolved in the alkaline electrolyte and then re-plated onto the electrode. The zinc does not necessarily re-plate back to the same place from which it was dissolved. This results in a redistribution of the zinc active material over the surface of the electrode which can cause lower cell capacity, reduced cycle life and premature cell failure.
Many factors influence the zinc redistribution process, known as shape change. These factors include cell design and construction, the type of separator used, thermal effects, electrolyte concentration gradients, current density distribution and others. The complex nature of the process and the many factors and variables which affect it, make it very difficult to control.
A number of approaches have been tried including a variety of electrode additives, electrolyte additives and separator types. Other more elaborate approaches involve electrode, cell, battery and system designs. Illustrative of these are circulating zinc-slurry type batteries and mechanically rechargeable batteries in which the spent zinc anode is physically replaced with fresh zinc.
Zinc electrode additives previously used include calcium-hydroxide and others. U.S. Pat. No. 3,516,862 issued to W. Van der Grinten, discloses adding calcium-hydroxide to the zinc-oxide active material mixture. Also U.S. Pat. No. 5,460,899 issued to Allen Charkey and assigned to the same assignee hereof, discloses a method of making a calcium-hydroxide/zinc-oxide electrode in a sealed nickel-zinc cell. In both cases, the zinc electrode is fabricated by combining calcium-hydroxide and zinc-oxide into the electrode mixture and forming calcium-zincate in situ in the alkaline electrolyte-activated cell.
Calcium-hydroxide in combination with zinc-oxide forms an insoluble phase known, and identified by x-ray diffraction, as calcium-zincate, having the structural formula CaZn(OH).sub.3 !.sub.2 .multidot.2H.sub.2 O. This relatively insoluble structure effectively ties up the zincate anion keeping it from diffusing into the bulk electrolyte where it is lost from the electrode. The calcium-zincate structure thus helps to hold the zinc active material in place on the electrode where it can be electrochemically utilized to provide energy storage capacity.
In prior approaches, calcium-hydroxide is added to the zinc-oxide active material during electrode fabrication. The electrode comprising the mixture of calcium-hydroxide and zinc-oxide is then incorporated into an alkaline electrolyte battery. Over the course of electrical charge/discharge cycling of the battery, the calcium-hydroxide, in combination with the zinc-oxide active material, then forms an insoluble phase of calcium-zincate, in situ. However, the conversion of the calcium-hydroxide and zinc-oxide to calcium-zincate is non-uniform, due to the effects of current density distribution, electrolyte concentration gradients and other factors.
There are additional disadvantages in using the in situ procedure for forming the calcium-zincate additive. One disadvantage is the dissolution of zinc active material into the battery electrolyte. This occurs during wetting down of the cell with electrolyte and during the initial cycles prior to complete conversion of the calcium-hydroxide to calcium-zincate. It results in loss of electrochemical zinc capacity. Another disadvantage is the changes that occur in the electrolyte concentration and distribution due to the consumption of water from the battery electrolyte. These disadvantages reduce battery charge/discharge cycle life.
It is, therefore, an object of the present invention to provide an improved zinc electrode active material containing calcium-zincate, and a method of making same.
It is a further object of the present invention to provide a zinc electrode and zinc battery incorporating such improved zinc electrode active material.